McCloskey’s Salt
McCloskey decides that he wants to make his
own table salt
Stoichiometry
Mass to Moles and Moles to Mass
He goes to the back room and cuts a piece of
sodium and weighs it. It has a mass of 5.67
grams. He puts it into a flask.
He’s a picky man, so you know you must get
the exact right amount of chlorine to place in
the flask for a perfect and COMPLETE reaction
of sodium and chlorine. No wasted material.
What mass of chlorine do you need?
Solution
Making NaCl(s)
Skeletal and Balanced Chemical Equation
How do sodium and chlorine combine to
make sodium chloride? In other words,
use what you’ve learned from balancing
equations.
2Na(s) + Cl2(g)
2 NaCl(s)
What type of reaction is this???
Yes, synthesis!
It’s all about the relationship
Problem solved?
Figure out your mole ratio between sodium and chlorine
2Na(s) + Cl2(g)
2 NaCl(s)
2Na(s) + Cl2(g)
The above reaction suggests that 2 moles of sodium
react with 1 mole of chlorine gas
Your ratio of Na : Cl2 is 2 : 1
This also means that Cl2 : Na is 1 : 2
2 NaCl(s)
5.67 g(Na)
x
1 g (Cl2)
2 g (Na)
=
WRONG!!!!
2.84 g (Cl2)
It’s a molar ratio/relationship, not mass
Every element has a different mass, so you can’t
compare mass...remember moles!!!
Welcome to Stoichiometry
Stoichiometry
From mass to moles, to moles, to mass
From mass to moles, to moles, to mass
2 Na(s) + Cl2(g)
2 Na(s) + Cl2(g)
5.67 g(Na)
mass (m)
2 NaCl(s)
5.67 g(Na)
mass (m)
5.67 g(Na) x 1 mol (Na)
= 0.247 mol
22.99 g (Na)
0.247 mol
moles (n)
2 NaCl(s)
0.247 mol(Na) x 1 mol (Cl2)
= 0.124 mol
2 mol (Na)
0.247 mol
moles (n)
0.124 mol
moles (n)
Stoichiometry
Stoichiometry
From mass to moles, to moles, to mass
2 Na(s) + Cl2(g)
5.67 g(Na)
8.79 g (Cl2)
mass (m)
0.247 mol
moles (n)
•
2 NaCl(s)
0.124 mol(Cl2) x
0.124 mol
70.90 g (Cl2)
1 mol (Cl2)
= 8.79 g
We always use the same type of information
to make the jumps between steps:
Molar mass of x
Molar mass of y
grams (x) ! moles (x) ! moles (y) ! grams (y)
Mole ratio from
balanced equation
moles (n)
More practice
Given: 4NH3 + 5O2 " 6H2O + 4NO
a) How many moles of H2O can be made
using 0.50 mol NH3?
b) what mass of NH3 is needed to make 1.5
mol NO?
c) how many grams of NO can be made
from 120.0 g of NH3?
Solutions
4NH3 + 5O2 " 6H2O + 4NO
a) # mol H2O= 0.50 mol NH3 x 6 mol H2O
= 0.75 mol H2O
4 mol NH3
b) # g NH =
3
1.5 mol NO x 4 mol NH3 x17.04 g NH3 = 26 g NH3
1 mol NH3
4 mol NO
c) # g NO=
120.0 g NH3 x 1 mol NH3 x 4 mol NO x 30.01 g NO
17.04 g NH3
4 mol NH3
1 mol NO
=
211.3 g NO
Homework:
Read Section 5.4 (p. 223 229)
Do problems 1 - 6 (p. 227)
and #9 (p. 228)